GMail is a browser e-mail management client that has existed since 2004. This Google's service is used by about a half of a billion of people from all around the world. While the service used to be rather minimalistic in its beginnings now it is packed with a dizzying array of other features and integrated with other Google's applications.
The most important change introduced by GMail is the messenger-style view. E-mails are grouped into threads, so when you exchange a couple of e-mails in the same topic they are displayed in the same window. This standard is nowadays used also in text message applications of Android and iOS.
Another important issue is archiving. Instead of removing your messages forever you can only sweep them out of your inbox and they go to the folder with all the e-mails. This way you can go back to them later. Another big advantage of this e-mail client is the search engine, since Google has utilized all its knowledge of designing great searching tools.
Among other additional Gmail's modules you should point out the chat. This little window in the left sidebar allows you to text your friends, along with conducting voice and video calls directly from the level of your e-mail browser client. This chat is now integrated with Google Hangouts.
Another great, however rarely used tools are: to-do list and contacts manager. The latter one is especially useful if you use an Android-running mobile phone. You numbers, e-mail addresses and other data is automatically synchronized, so you can access your contacts in a new smartphone right after connecting to the Internet.
In the left sidebar you can also find other e-mail folders (such as Bin and Spam) along with so-called labels. Labels allow you to mark your e-mails and assign priority. You can do it either automatically (by creating filters) or manually. Filters are also useful in sorting and eliminating e-mails from certains senders.
You can further improve Gmail's funcitonality with a vast variety of different settings. In Google Labs you can also find some experimental features and add-ons. Google's GMail display context ads that are fortunately not too invasive and don't slow your browser's performance. You can easily disable them by using an add-on such as the AdBlock.